This was originally build for my grade-school girls to talk to each other on the telephone between their bedrooms.
Basically it is a two-jack phone wall plate modified to act as the connecting point for two extension phones. The plate ties the two 'lines' together and supplies power. Two phones are connected to their respective jacks. Taking the phones 'off hook' will allow the batteries to power the circuit and you can carry on a conversation.
The batteries are connected in series to provide 18V to power the phones. The battery life is not that great as I recall, so it may be worthwhile to investigate using an old laptop plug-in power supply to power it. I understand that some very old phones may have polarity sensitivity so if the circuit does not work try reversing the battery connection to the wall jacks.
The schematic shows the connections. Don't forget the 300 ohm resistor as shown. A call buzzer or light could be added with additional components and wiring to make the setup more useful -- but for two pre-teens between bedrooms this was sufficient.